Activist shareholder Elliott Advisors is calling on BHP Billiton PLC (BLT.LN) management to undertake a full, independent and transparent review of unification at the dual-listed mining firm by Feb. 20, when it reports first-half earnings. In a letter on Monday to BHPChairman Ken MacKenzie, Elliott said that unification will deliver more than $22 billion in value to shareholders and cost only $391 million...